A power supply provides a desired current and/or voltage to a load. For example, referring to FIG. 1, a power supply 10 receives input power from an AC power source 12 in the form of an input voltage vin and an input current iin and converts the input voltage to a desired output voltage Vout. The output power is supplied to a load 14 with a current Iload. Accordingly, the function of the power supply 10 is to take the input from the AC source and convert it to a DC or AC current/voltage for the load 14.
The load 14 may, for example, operate from a DC or AC input voltage. An example of a load that may be advantageously operated using a DC input voltage is a solid state light emitting apparatus that includes one or more solid state light emitting diodes (LEDs). Many different types of loads can be powered by a DC input voltage, such as electronic circuits, battery chargers, etc. Other loads, such as electric motors, are typically driven by AC input voltages having suitable amplitudes and frequencies.
FIG. 2 shows a simplified circuit diagram of a power supply 10 that includes an electromagnetic interference (EMI) reducing filter 20 and a bridge rectifier 22 followed by a bulk capacitor CB. The bridge rectifier 22 provides a fully rectified signal to the bulk capacitor CB. A DC voltage is obtained across the bulk capacitor CB, which is then used to power the load 14. Because of the DC voltage across the bulk capacitor CB, the load 14 may draw current from the source only at the peak of the line voltage vin when the line voltage vin is higher than the DC voltage across the bulk capacitor CB.
FIG. 3 is a graph of the input (line) voltage vin and the input current iin for the power supply 10 shown in FIG. 2. As shown in FIG. 3, the input current is pulsating, which may result in rich harmonic contents in the power supply and cause the circuit to have a poor power factor (PF). Power factor is the ratio of the real power flowing to the load to the apparent power in the circuit. A load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. These higher currents increase the energy lost in the distribution system, reducing the efficiency of the power supply.
One way to address this problem is to employ a so called active power-factor correction (PFC) circuit, an example of which is shown in FIG. 4. Referring to FIG. 4, a line voltage vin is filtered by an EMI filter 20 and rectified by a bridge rectifier 22. The filtered, rectified signal is then provided to a boost power supply 30.
The boost power supply 30 with active PFC includes a boost inductor LPFC, a diode D5, a bulk capacitor CB and a switch Q1.
The switch Q1 controls the flow of current through the PFC inductor LPFC, and is itself controlled by a controller 32. When the switch Q1 is turned ON, current through the PFC inductor LPFC increases rapidly, causing magnetic energy to be stored in the PFC inductor LPFC. In particular, when the switch Q1 is turned on, the PFC inductor current iLPFC ramps up at a rate of VREC/LPFC, where VREC is the rectified voltage of the line voltage vin.
When the switch Q1 is turned OFF, energy stored in the PFC inductor LPFC is output through the diode D5 in the form of current that charges the bulk capacitor CB. In particular, when switch Q1 is turned off, the voltage across the switch Q1 increases to a level higher than output voltage VB on the bulk capacitor CB, so that the diode D5 conducts. The inductor current iLPFC decreases at a rate of (VO-VREC)/LFPC.
The bulk capacitor CB provides an output current ILED to the LED load 14. The controller 32 monitors the current flowing in the load based on a voltage through a sense resistor RS and controls the state of the switch Q1 to achieve a desired power factor.
FIG. 5 is a graph of the input (line) voltage vin and the input current iin for the power supply 30 shown in FIG. 4. As shown in FIG. 5, the line current iin is controlled to follow the shape of the line voltage, which may result in a higher power factor and/or reduced harmonic current contents in the power supply.
In general, there are three control modes for PFC circuits. The first is the discontinuous-conduction mode (DCM); the second is the continuous-conduction mode (CCM); and the third is the critical/boundary CCM/DCM mode. In DCM, during each switching cycle, the inductor current iLPFC falls to zero sometime before the switch Q1 is turned on, and the current of the diode D5 also falls to zero naturally. Therefore, in DCM there is no reverse recovery loss associated with the diode D5.
In CCM, during each switching cycle, the inductor current iLPFC as well as the current through the diode D5 do not fall to zero before switch Q1 is turned on. Therefore, in CCM, there is reverse recovery loss associated with the diode depending on the property of the diode. In DCM/CCM boundary/critical mode, the switch Q5 is turned on right after the inductor and diode current falls to zero, resulting in no reverse recovery loss.
FIG. 6 shows the current waveform of a PFC inductor operating in DCM/CCM boundary mode. In particular, FIG. 6 illustrates the rectified input voltage vREC, the inductor current iLPFC and the input line current iin for a PFC inductor operating in DCM/CCM boundary mode. It can be seen from FIG. 6 that the peak current of the PFC inductor is twice the absolute line current iin. Although there is no reverse recovery loss associated with PFC diode D5 in this case, significant power loss may result in the PFC inductor winding and switch Q1 because of the severely pulsating current. Furthermore, the pulsating current needs a large EMI filter, increasing the size and cost of the power supply.
For CCM PFC control, the inductor current ramps up and down following the shape of the input voltage. FIG. 7 shows the current waveform of a PFC inductor operating in CCM. In particular, FIG. 7 illustrates the rectified input voltage vREC, the inductor current iLPFC and the input line current iin for a PFC inductor operating in CCM. As shown in FIG. 7, the ripple of the PFC inductor current iLPFC is significantly lower, depending on the inductance of the PFC inductor and switching frequency. A lower ripple current of the PFC inductor leads to a smaller EMI filter and lower conduction power loss in the PFC inductor LPFC and switch Q1.
FIG. 8 shows yet another power supply 40 including a boost PFC 42 and a DC/DC converter 44. The PFC stage 42 shapes the input current waveform so that it follows the input voltage waveform. The DC/DC stage 44 converts the PFC output voltage VB to a suitable voltage/current to drive the LED load 14. A dimming controller 46 controls the brightness of the LEDs with pulse-width-modulated dimming or analog/linear dimming.